docs: def update(self, callback=int):
          """Process all pending model modifications."""
          # print(self._pending_modifications)
          add_var = self._pending_modifications.add_var
          if len(add_var) > 0:
              self._add_variables(add_var)
              self._pending_modifications.add_var = []
          callback()
  
          add_constr = self._pending_modifications.add_constr
          if len(add_constr) > 0:
              self._add_constraints(add_constr)
              self._pending_modifications.add_constr = []
  
          add_constr_sloppy = self._pending_modifications.add_constr_sloppy
          if len(add_constr_sloppy) > 0:
              self._add_constraints(add_constr_sloppy, sloppy=True)
              self._pending_modifications.add_constr_sloppy = []
  
          var_lb = self._pending_modifications.var_lb
          var_ub = self._pending_modifications.var_ub
          if len(var_lb) > 0 or len(var_ub) > 0:
              self._set_variable_bounds_on_problem(var_lb, var_ub)
              self._pending_modifications.var_lb = []
              self._pending_modifications.var_ub = []
  
          rm_var = self._pending_modifications.rm_var
          if len(rm_var) > 0:
              self._remove_variables(rm_var)
              self._pending_modifications.rm_var = []
          callback()
  
          rm_constr = self._pending_modifications.rm_constr
          if len(rm_constr) > 0:
              self._remove_constraints(rm_constr)
              self._pending_modifications.rm_constr = []